1. Field of the Invention
The present invention relates to a system for handling of articles such as baggage or carrier cargo. In particular the invention relates to a system for automated identification of articles wherein an electronic sub system called an interrogator including a transmitter and receiver extracts by electromagnetic means useful information from an electronically coded label attached to such articles as they are processed through sorting operations eg. at an airport or node of a cargo handling organisation.
Although the present invention is herein described with reference to a baggage/cargo sorting system it is to be appreciated that it is not thereby limited to such applications. Thus the sorting system of the present invention may be applied to material handling operations generally eg. sorting of stock or parts.
2. Description of the Prior Art
A block diagram of the type of system to which the invention relates is shown in FIG. 1. This system uses the principle of electromagnetic communication in which an interrogator containing a transmitter generates an electromagnetic signal which is transmitted via an interrogator antenna system to an electronic label containing a label receiving antenna. The label antenna receives a proportion of the transmitted energy and through a rectifier generates a dc power supply for operation of a reply generation circuit connected either to the label receiving antenna or a separate label reply antenna with the result that an information bearing electromagnetic reply signal is radiated by the label.
As a result of electromagnetic coupling between the label and interrogator antennae, a portion of a time-varying radio frequency signal is transmitted by the label antenna and may enter the interrogator antenna, and in a signal separator located within the interrogator be separated from the signal transmitted by the interrogator, and passed to a receiver wherein it is amplified, decoded and presented via a microcontroller in digital or analog form to other systems such as a host computer or a system of sorting gates which make use of the information provided by the interrogator.
In the label, operations of the reply generation circuit may be controlled in time by an oscillator, the output of which may be used either directly or after reduction in frequency by a divider circuit to control the code generator circuit and a reply interval generator circuit. The code generator circuit may control a modulator circuit which may present a time-varying impedance varying in accordance with the modulation signal either directly to the receiver antenna or to the rectifier, or may present a modulated reply carrier signal to a reply antenna. The code generator circuit may alternatively present to the receiver antenna or to the rectifier a reply signal without carrier wave. The reply interval generator circuit may control timing signals to the code generator circuit or to the modulator circuit so that the reply signal is radiated by the label for only a portion of the time for which the label is interrogated.
Propagation of electromagnetic signals between the interrogator antenna system and the label antenna may be constrained to take place within a field confinement structure which may be used to enhance the coupling of energy between the interrogator antenna system and the label antenna, and may also be used to diminish unwanted propagation of interrogator energy beyond the region desired for interrogation. The interrogator antenna system may be connected to the interrogator via an antenna re-configuration switch, either mechanical or electronic, which allows the nature of the interrogation field created by the interrogator antenna system at the position of the label to be changed in magnitude and direction. Such antenna re-configuration may be automatic over time or may be under control of the microcontroller within the interrogator.
Within the interrogator the transmitter may generate, in addition to the signals supplied to the interrogator antenna system, reference signals supplied to the receiver and may also generate signals supplied to a field cancellation system which may be placed externally to the field confinement system or to the region occupied by the interrogator antenna system, and may serve to reduce the net propagation of interrogation signals beyond the region desired for interrogation. The signals supplied to the field cancellation system may be fixed in nature or may be varied under control of the microcontroller which may receive signals from a field sensing system which samples the unwanted propagating signals in regions external to the interrogation region.
Within the interrogator the microcontroller may perform in addition to the operations discussed various calculation and control functions such as functional testing of the system components and may also participate in the reply decoding process.
In the design of practical systems for cargo and baggage handling several problems can arise. One problem is that of discrimination between replies which can come simultaneously from identification labels attached to different objects simultaneously present in the interrogation field. The usual solution to this problem is to ensure that the interrogation field is created at a very low frequency, below the broadcast band, by a near-field dipole antenna in which the interrogation field strength diminishes as approximately the third power of the distance from the antenna to the label, and to simultaneously ensure that differently labelled objects are sufficiently separated in their passage through the identification apparatus for the field of the interrogator to excite only one label at a time. When such sufficient separation is not provided, replies from different tags interfere one with another, and incorrect readings, or a failure to read a label, can occur.
A further problem is caused by orientation sensitivity of magnetic field sensitive label antennae to low frequency interrogation fields. The problem is compounded by unpredictability of construction of objects to which labels are to be attached, and the modification to the field created by the interrogator which can occur in commonly occurring situations. An example is provided by the metal clad suitcase, upon the surface of which eddy currents are created by the interrogation field, so that the resulting oscillating magnetic field in the vicinity of the surface is constrained to lie parallel to that surface. This field re-orientation, together with the fact that planar coil label antennae are insensitive to a magnetic field within their plane, causes low frequency labels which lie close to and parallel to such metal surfaces not to respond. It is also the case that simply configured magnetic field creating antennae generate field configurations with symmetry planes through which a conveyor borne label can pass without at any stage achieving strong coupling to the interrogation field.
A further problem is that of achieving an interrogation field which provides an acceptably low level of interference to other users of the electromagnetic spectrum. Commonly used solutions to this problem consist of either the use of interrogation frequencies well below the broadcast band, at which frequencies radiation restrictions are generally less stringent than at higher frequencies, and for which radiation from magnetic field producing antennae of size useful in interrogation of labels attached to person-portable objects is small, or the use of UHF or microwave frequencies where in some countries bands allowing greater stray radiation are defined.
However, each of these solutions has disadvantages in respect of both label manufacturing cost and performance. When interrogation frequencies below the broadcast band are used, label antennae require finely etched patterns of many turns to achieve the required induced voltages, and generally also require installation in the label of resonating capacitors which are of a size impractical either to apply separately at low cost or to manufacture in monolithic integrated circuit form. Moreover the use of such low frequencies renders the weak label replies which also occur at low frequency difficult to distinguish against a relatively high level of electromagnetic background noise present in all urban environments.
When UHF or microwave frequencies are used, circuit components which will perform all the functions required to extract energy from the interrogation signal and generate a reply become impossible both to manufacture inexpensively and to incorporate into a single integrated circuit manufacturing process, with the result that label manufacturing costs again become unacceptable for mass application. At these frequencies there is the additional problem that reflections of interrogation energy and multi-path propagation can generate concentrations of interrogation energy at regions outside the intended interrogation region, and ambiguities of the source of a reply can occur. Such frequencies also suffer from both screening by well-conducting objects and attenuation by partially conducting objects when the identifying label happens to be positioned with an obstructed view of the interrogation antenna.
The problems described above are compounded when the objective of obtaining from licensing authorities type approval rather than individually licensed approval of automated identification and sorting installations, particularly in a sensitive location such as an airport, is considered. For type approval, radiation of the interrogation energy beyond the interrogation area is required to be kept at a particularly low level, while at the same time, because the label contains no energy source so that manufacturing costs may be kept low, the interrogation field is required to be strong enough to allow the label to derive its operating power therefrom. These two requirements are substantially in conflict.
Further problems arise from the weak reply which occurs in passive labels as just described. In the presence of a weak reply, there is a need to prevent extraneous noise from the environment from entering the receiver where it may mask the reply. There is a need also to prevent excessive amounts of the strong interrogation signal from entering the receiver wherein it may either do damage, or through saturation of mixer or amplifier elements, reduce sensitivity.
The usual method of keeping excessive interrogation power from entering the receiver is to employ a transmitter-receiver signal duplexer in the form of a directional coupler, a circulator or a bridge circuit. Such circuit elements only provide the necessary degree of isolation when carefully adjusted to achieve an appropriate match between the input impedance of the antenna structure, and a characteristic impedance of the duplexer structure. Even when such isolation is achieved by careful adjustment, introduction of objects to be sorted of significant size within the field of the interrogator antenna system will change its impedance properties by a sufficient amount to destroy the isolation achieved. The problem is not significantly alleviated by using separate transmitter and receiver elements because the factors just mentioned make maintenance of a high degree of isolation between such elements impossible to maintain.
As well as producing a loss of sensitivity of the receiver due to saturation, lack of sufficient isolation between transmitter and receiver paths can produce other undesirable effects. It is frequently the case that in order to enhance the coupling between label and interrogation apparatus, a single label antenna operating over a relatively narrow frequency band is used, and the reply signals occupy a portion of the electromagnetic spectrum close to the transmitted spectrum. In this situation phase noise inevitably present in the transmitter signal will appear as background noise in the receiver channel, and will provide an unwelcome limitation to sensitivity.
Further problems arise in the receiver structure normally used as a means of avoiding effects of low frequency phase noise in either transmitter or receiver local oscillator. The receiver structure normally used is the homodyne receiver in which the same oscillator is used as the primary generator of transmitted signal and as a local oscillator for the down-converting mixer in the receiver. Such receivers have the benefit of avoiding the effects of low frequency phase noise which arise when separate transmitter and receiver local oscillators are used.
However, homodyne receivers have the property that the phase between the reply signal and the local oscillator presented to the down-converting mixer is relevant to performance, in that when those two signals are in quadrature, there is no mixer output in the desired output passband. When very low interrogation frequencies are used, the requirement to maintain an appropriate phase relationship between the reply signal and the receiver local oscillator can be achieved by initial design and adjustment of the interrogator circuits. As, however, interrogation frequency rises toward the higher values desired to allow low-cost processes to be employed in manufacture of labels, phase shifts, due to the movement of objects to be identified within the scanning field and time delays in the propagation path between the label and interrogator antennas, make the phase of the reply reaching the receiver mixer uncertain, and positions of the label occur for which effectively no reply is seen. Measures need to be taken to deal with this matter.
The usual measure is to split the reply signal into two halves which are down-converted in frequency in separate mixers, the local oscillator signals fed to those mixers being in phase quadrature. The output signals from the mixers are then either re-combined after one is modified to suffer a further phase shift over the relevant band, or are separately decoded, and compared in amplitude and in the decoded result, before a decision is made on whether a correct reply has been received. Each of these measures has its shortcomings, either in distortion of the signal envelope in the in-phase and quadrature channel recombination process, or in introducing additional complexity to the interrogator through the requirement to provide circuits for processing of split signals down to and including the decoding operation.